Chambers which have end surfaces being displaceable free of leakage and which vary the volume by varying the direct action against this end surface, are used for different purpose such as transmission of power, pressure metering, switching functions and such. The relevant chambers in this connection may be corrugated bellows produced in metal or in a soft material, or cylindric chambers where the change of volume occurs by displacement of an elastic membrane or a piston with sliding seal. In many connections it is desirable that the volume of the chamber may be effected by only small changes in the load balance effecting the displaceable end surface. This may easily be achieved with the difference between the internal and external pressure of the chamber can be kept at a low level. It becomes substantially more difficult when the difference between the internal and external pressure exceeds for example 250 bar. If a corrugated bellow is to withstand pressure differences of this magnitude, it must be dimensioned very strongly and a significant change of volume therefore will require a large change in the direct action. Correspondingly a membrane or a traditional soft sliding seal will be squeezed if they are exposed for large pressure differences and only small changes of the volume can be achieved before the seals are damaged. Different high pressure sliding seals are produced of a suitable material, but they have a large sliding friction and therefore limited range of use.